The present invention relates to an optical switch, and more particularly to an optical switch used in the fields of optical fiber communication, optical information processing, and the like.
Recently, all-optical systems has been widely proposed for optical fiber communications and optical information processing. The research and development is being vigorously conducted to realize the all-optical systems. All-optical systems mean that signals propagate in the form of light through the transmission path, the multiplex/separation circuit, logic circuits, and the like within the system while not being subjected to a light-to-electric signal conversion or an electric signal-to-light conversion during the propagation.
Such an all-optical system requires elements controlling optical signals at high speeds. Conventionally, the method of performing optical control by electric signals has been employed for the optical control. However, an optical control by light (that is, all-optical control) is being recently taken notice of as a method of promising higher speed operation.
Particularly, the optical communication system where an ultra high-speed optical switch controlled by light (all-optical switch) is applied to the optical demultiplexer will be a breakthrough to realize a high-speed optical communication system.
In the all-optical switch, the most important performance is a high speed characteristic. Similarly, other performances are required regarding to low switching energy, high repetitive operation, compact size, and others.
Particularly, the switching energy is required to be within the light pulse energies deliverable from a semiconductor laser, fiber amplifier, or semiconductor light amplifier.
In order to realize those performances, the critical matter is the empirical rule that the figure of merit .chi.(3)/.tau..delta. of a non-linear optical effect, being the drive principle of an all-optical switch, is nearly constant, where .chi.(3) is the size of the non-linearity, .tau. is a response time and .delta. is a signal loss.
The non-linear optical effect is roughly classified into a non-resonance excitation type and a resonance excitation type. It has been considered that it is difficult for either type to provide both high efficiency and high-speed. The non-resonance excitation type has a possible high-speed operation but the efficiency thereof is low.
Hence, a high-switching energy or a long interaction length is required. In contrast, the resonance type has a high efficiency but the relaxation of electrons actually excited in a non-linear optical medium is low. This problem makes it difficult to realize a high-speed operation.
The high efficiency of the resonance excitation type has been gaining renewed interest because of holding large appeal in a practice use. Recently, various methods have been proposed to overcome the problem on the slow relaxation. JP-A-No. 115844/1998, for instance, discloses an all-optical switch using the non-linear optical effect of the resonance excitation type.
The operation of an all-optical switch using the non-linear optical effect of the resonance excitation type will be described below. FIG. 5 is a block diagram illustrating the configuration of the optical switch, or all-optical switch, of FIG. 5. FIG. 6A is a waveform diagram of signal light to the port 11. FIG. 6B is a waveform diagram of signal light to the port 11. FIG. 6C is a waveform diagram of controlling light from the port 12.
A non-linear optical waveguide 1 receives a signal light pulse of a frequency of .omega.1 and a controlling light pulse of a frequency of .omega.0 through a signal light/controlling light input port 11. The frequency .omega.1 of the signal light is set to the transparent region of the non-linear optical waveguide 1. The non-linear optical waveguide 1 absorbs or amplifies the controlling light and changes the non-linear refractive index of the signal light.
As a result, the signal light is subjected to a non-linear phase shift and an optical frequency shift due to the non-linear phase shift in the non-linear optical waveguide 1. The non-linear optical waveguide 1 guides the signal light to the signal optical output port 12 via the optical frequency filter 2. The optical frequency filter 2 is set to pass the signal light pulse of which the center frequency is shifted by .omega.2.
A change in non-linear refractive index occurs by absorption or amplification of the controlling light within the non-linear optical waveguide 1. The controlling light absorption results in an increase in carriers in the non-linear optical waveguide. The controlling light amplification results in a decrease in carriers in the non-linear optical waveguide. In either case, a change in non-linear refractive index occurs by the number of carriers, thus resulting in a non-linear phase shift of the signal light.
A typical time characteristic to a non-linear phase shift (.DELTA..phi.) due to the resonance excitation contains an ultra high-speed rise following a controlling light pulse and a slow fall due to the carrier recombination.
Let us now consider a change in non-linear refractive index due to the controlling light absorption. In the case of the controlling light absorption type, the non-linear phase shift is approximately proportional to the number of carriers in the non-linear optical waveguide.
Hence, the time characteristic of the non-linear phase shift .DELTA..phi. is expressed by the following rate equation: EQU d(.DELTA..phi.)/dt=G-d(.DELTA..phi.)/.tau. (1)
where G is an amount proportional to an instantaneous strength of a controlling light pulse and .tau. is a relaxation time.
The time differential of a non-linear phase shift provides a frequency shift (.DELTA..omega.) of the signal light. EQU .DELTA..omega.=d(.DELTA..phi.)/dt (2)
When the non-linear phase shift occurs by excitation of a short controlling light pulse, only the first term of the rate equation contributes to the frequency shift and is expressed by the following equation. EQU .DELTA..omega.=d(.DELTA..phi.)/dt.about.G (3)
This means that a large frequency shift is provided only to the signal light incident together with the controlling light pulse. In the case of, for instance, controlling light with a pulse width of 3 ps, a large frequency shift is effected to the signal light for 3 ps. Since the falling time normally is on the order of ns, the frequency shift due to relaxation of a change in refractive index is two or three figures smaller than the frequency shift in the rise time of a change in refractive index.
The operation of the conventional optical switch will be described with reference to FIG. 6A, FIG. 6B and FIG. 6C.
A chain of signal light pulses to be input is obtained by data modulating a pulse chain of 160 GHz and with a pulse width of 2 ps. The chain of controlling light pulses has a pulse width of 3 ps and a repetitive frequency of 10 GHz.
The induced non-linear phase shift for the signal light pulse is 2.pi.. The frequency shift is provided only to the signal light pulse simultaneously input together with the controlling light pulse. The center frequency of the spectrum of frequency-shifted signal light pulses is .omega.2.
Under the above operational conditions, the shift ranging .omega.1 to .omega.2 is about 0.5 THz. The optical frequency filter 2 passes only the signal light pulse of which the center frequency is shifted to .omega.2 triggered by the controlling light pulse, to the output port 12. This feature allows a high-speed, high-efficiency full optical switch to be realized in a simplified configuration.
Both sides of the second equal sign of the equation (3) are integrated over time. Thus, it may be concluded that the non-linear phase shift .DELTA..phi. excited by the controlling light is proportional to the amount G integrated over time, that is, a controlling light pulse energy.
The control optical pulse energy is considered by estimating a non-linear phase shift necessary for a switching operation. As understood from the first equal sign of the equation (3), the non-linear phase shift is expressed by the following formula. EQU .DELTA..phi..about..DELTA..omega..times.t0 (4)
The non-linear phase shift is determined by a frequency shift to be provided for a signal light and by a controlling light pulse width.
The frequency shift necessary for an switching operation is set to a larger value than the spectrum width .DELTA..omega.1 of a signal light such that the optical frequency filter can identify a frequency-shifted component from the other components. EQU .DELTA..omega.&gt;.DELTA..omega.1 (5)
In the previous optical switch, the control light pulse width is set to a larger value than a signal light pulse width. EQU t0&gt;t1 (6)
Hence, the formula (4) is expressed as follows: EQU .DELTA..phi.&gt;.DELTA..omega.1.times.t1 (7)
The time waveform of a light pulse is combined with the spectrum waveform through a Fourier transform. It is well known that the product of the pulse width t1 of a signal light pulse and the spectrum width .DELTA..omega.1 does not become less than a constant value. The pulse that provides a minimum product of a pulse width and a spectrum width is called a Fourier-transform limit pulse.
The inequality (7) expresses that the non-linear phase shift does not become less than the value obtained by substituting t1 and .DELTA..omega.1 under the Fourier-transform limit condition of a signal pulse.
In an application of the optical switch to the optical separation circuit at the receiving terminal of the optical communication system, it must be considered that the signal light pulses propagating in a fiber are influenced by the group velocity dispersion and the non-linear optical effect and then are input to the optical switch.
In other words, it is considered that the pulse width t1 and the non-linear spectrum width .DELTA..omega.1 of a signal light pulse spread during the propagation in a fiber so that the non-linear phase shift is deviated from the Fourier-transform limit condition.
In such a case, the non-linear phase shift necessary for the switching operation increases, as understood from the inequality (7). This means that the controlling light pulse energy increases.